1. Field of the Invention
The present invention relates to a display apparatus, a method for driving the display apparatus and electronic equipment. More particularly, the present invention relates to a display apparatus of a flat-panel type, in which pixel circuits each including an electro-optical device are laid out to form a matrix, a method for driving the display apparatus and electronic equipment employing the display apparatus.
2. Description of the Related Art
In recent years, in the field of a display apparatus for displaying an image, a display apparatus of a flat-panel type, in which pixels (or pixel circuits) each including a light emitting device are laid out to form a matrix, has been becoming popular very fast. A light emitting device included in each pixel circuit in the display apparatus of a flat-panel type is an electro-optical device of the so-called current-driven type in which the luminance of a light beam emitted by the device changes in accordance with the magnitude of a current flowing through the device. The development of an organic EL (Electro Luminescence) display apparatus employing such electro-optical devices into a commercial product has been making progress. An example of the electro-optical device of the so-called current-driven type is an organic EL device operating on the basis of a phenomenon in which a light beam is generated by the device when an electric field is applied to an organic film.
The organic EL display apparatus has the following characteristics. The organic EL device employed in the EL display apparatus can be driven by an applied voltage not exceeding 10V so that the power consumption of the device is low. In addition, since the organic EL device is a light emitting device, the organic EL display apparatus is capable of displaying an image which is visible in comparison with a liquid crystal display apparatus for displaying an image by controlling the intensity of a light beam generated by a light source known as a backlight in a liquid crystal cell included in every pixel circuit of the liquid crystal display apparatus. On top of that, the organic EL display apparatus can be made light and thin with ease because the organic EL display apparatus does not need illumination members such as the backlight which is necessary for the liquid crystal display apparatus. Furthermore, the organic EL device has an extremely high speed providing a short response time of the order of several microseconds. Thus, a residual image is not generated in an operation to display a moving image.
Much like the liquid crystal display apparatus, a passive matrix method or an active matrix method can be adopted as a method for driving the organic EL display apparatus. However, even though an organic EL display apparatus adopting the passive matrix method has a simple structure, the apparatus raises problems such as difficulties to implement a large display screen having a high resolution. For the reasons described above, an organic EL display apparatus adopting an active matrix method is developed aggressively. In accordance with this active matrix method, an active device is provided in the same pixel circuit as an electro-optical device. The active device is used for controlling a current flowing through the electro-optical device. An example of the active device is an insulated-gate type field effect transistor which is generally a TFT (thin film transistor).
By the way, the I-V characteristic (that is, the current-voltage characteristic) of an organic EL device is known to deteriorate with the lapse of time in the so-called aging process. In a pixel circuit employing an N-channel TFT for controlling a current flowing through the organic EL device, the organic EL device is connected to the source of the transistor which is referred to hereafter as a driving transistor. Thus, when the I-V characteristic of the organic EL device deteriorates, a voltage Vgs appearing between the gate and source of the driving transistor changes. As a result, the intensity of a light beam generated by the organic EL device also changes as well.
To put it more concretely, an electric potential appearing at the source of the driving transistor is determined by the operating points of the driving transistor and the organic EL device. When the I-V characteristic of the organic EL device deteriorates, the operating points of the driving transistor and the organic EL device change. Thus, the electric potential appearing at the source of the driving transistor also changes even if a voltage applied to the gate of the transistor after the operating points of the driving transistor and the organic EL device change is sustained at the same level as that before the operating points of the driving transistor and the organic EL device change. Accordingly, the voltage Vgs appearing between the gate and source of the driving transistor also changes as well, causing a current flowing through the transistor and a current flowing through the organic EL device to vary. As a result, since the current flowing through the organic EL device varies, the intensity of a light beam generated by the organic EL device also changes as well.
In addition, in the case of a pixel circuit employing a poly-silicon TFT, not only does the I-V characteristic of the organic EL device deteriorate with the lapse of time, but the threshold voltage Vth of the driving transistor and the mobility μ of a semiconductor film composing the channel of the transistor also change with the lapse of time. In the following description, the mobility μ of a semiconductor film composing the channel of a driving transistor is referred to as the mobility μ of the driving transistor. On top of that, the threshold voltage Vth and mobility μ of the driving transistor each vary from pixel to pixel due to variations in fabrication process. That is to say, the characteristic of the driving transistor varies from pixel to pixel.
If the threshold voltage Vth and mobility μ of the driving transistor each vary from pixel to pixel, the current flowing through the transistor also varies from pixel-to-pixel. Thus, the luminance of a light beam generated by the organic EL device also varies from pixel to pixel even for the same voltage applied to the gate of each driving transistor. As a result, the screen loses uniformity.
In order to prevent the luminance of a light beam generated by the organic EL device from varying from pixel to pixel even for the same voltage applied to the gate of each driving transistor and, hence, from being affected by deteriorations of the I-V characteristic of the organic EL device and/or changes of the threshold voltage Vth and mobility μ of the driving transistor even if the I-V characteristic deteriorates with the lapse of time and/or the threshold voltage Vth and the mobility μ change with the lapse of time, it is necessary to provide every pixel circuit with a compensation function and a variety of correction functions as is described in documents such as patent reference 1 which is Japanese Patent Laid-open No. 2006-133542. The compensation function is a function to compensate for characteristic variations of the organic EL device. The correction functions include a threshold-voltage correction function and a mobility correction function. The threshold-voltage correction function is a function to make corrections for threshold voltage (Vth) variations of the driving transistor. On the other hand, the mobility correction function is a function to make corrections for mobility (μ) variations of the driving transistor.
As described above, every pixel circuit is provided with the compensation function to compensate for characteristic variations of the organic EL device, the threshold-voltage correction function to make corrections for threshold voltage (Vth) variations of the driving transistor and the mobility correction function to make corrections for mobility (μ) variations of the driving transistor. Thus, it is possible to prevent the luminance of a light beam generated by the organic EL device from varying from pixel to pixel even for the same voltage applied to the gate of each driving transistor and, hence, from being affected by deteriorations of the I-V characteristic of the organic EL device and/or changes of the threshold voltage Vth and mobility μ of the driving transistor even if the I-V characteristic deteriorates with the lapse of time and/or the threshold voltage Vth and the mobility μ change with the lapse of time.